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The association of major, minor and trace inorganic elements with lignites. II. Minerals, and major and minor element profiles, in four seams
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The association of major, minor and trace inorganic elements with lignites. II. Minerals, and major and minor element profiles, in four seams ROBERT N. MILLER* and PETER H. GIVEN** College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, PA. 16802, U.S.A (Received October 16, 1985; accepted in revised,form February 16, 1987) Abstract-Inorganic elements in lignites may be present in carboxylate salts and organic coordination complexes as well as in discrete mineral phases. Moreover, organic matter can alter mineral phases in peats. The objectives of this study are to investigate how a number of important elements are held in some typical lignites and hence to document organic/inorganic interactions of various kinds. Accordingly, detailed profiles of the distributiop of IO major and minor elements through four lignite seams have been made, including roof and floor rocks and any mineral partings. Elements extractable by 1.0 N ammonium acetate and by 1.O N HCl and in the insoluble residue have been analyzed by plasma arc emission and atomic absorption spectrometry. Mineral phases were analyzed by x-ray diffraction and infrared spectrometry. Substanial proportions (40-90%) of the Ca, Mg, Sr, Ba, Na and Mn were extractable from the organic zones, but not the inorganic, by ammonium acetate, and were inferred to have been present in the organic matter as carboxylate salts. Much of the K and some Mg were present in clay minerals, but K-containing clays were depleted in the coal layers relative to roof and floor, probably because organic acids in the peat had leached the K out of illite, thus altering the clays. Part of the Mn was in the organic matter either complexed or dispersed as acid-soluble mineral (e.g. MnS, MnCO,). Acid-soluble Al and Ti were found to be quite abundant in the organic layers but not in the roof and floor; they probably are complexed with the organic matter. The principal conclusion is that the organic matter in peat and lignites has a profound influence, in many ways, on the suite of inorganic materials with which it becomes associated. INTRODUCTION COAL ISA SEDIMENTARYrock containing both organic and inorganic materials. In the case of coals of high volatile bituminous and higher rank the inorganic materials are largely discrete mineral phases. Peats and low rank coals contain 1-4 meg/g of carboxylic acid groups, which can bind cations in salt form. These and other functional groups, can also hold cations as coordination complexes. Such complexes, particularly if they involve chelation, can have (negative) free energies of formation of many tens of kilocalories per mole (SILL~N and MARTELL, 197 l), whereas ion exchange processes involve the release of only 0.2-2 kcal./mole (GIVEN and DICKINSON, 1975 and references therein). Cations held in these ways can constitute 50% or more of the ash-forming constituents in a lignite. It is known that organic materials can alter certain detrital mineral grains (e.g. ilmenite, illite: CAROL, 1960; TEMPLE, 1966; EHRLICH, 1981; BAILEY and KOSTENS, 1983). The presence of various microorganisms can promote the deposition of such authigenic minerals as pyrite, carbonates and opaline silica (BROCK, 1984; EHRLICH, 1981; ANDREJKO et al., 1983). These various processes will have occurred at the peat stage, but the effects will remain after coalification to high rank. Thus the distribution of inorganic matter in coals presents some interesting problems on the borderline between organic and inorganic geochemistry, and these problems will be especially chal-
Present addresses: * Air Products& Chemicals,Inc., P.O. Box 538, Allentown, PA 18105. ** Calda Cottage, 5 High
Street, South Woodchester, Stroud, Glos., GL5 5EL, England.
lenging when peats and low rank coals are considered, since with them a wider variety of phenomena will be important, involving interaction between organic and inorganic substances. It is the purpose of the series of papers of which this is the second to address these problems as they arise in the study of five lignites from various basins in North America. We have already published an account of our procedures and their application to a column sample of a North Dakota lignite seam (MILLER and GIVEN, 1986). We offer, here, data for four lignites from other localities, which were available as drill cores including the roof and floor rocks. Much published work on inorganics in coals (reviewed by MILLER and GIVEN, 1986; RENTON, 1982) has been restricted to samples of high volatile bituminous and higher rank; the nature of any association with organic matter is generally not discussed. A number of recent publications do deal with certain aspects of the occurrence of inorganic matter in peats and lignites (e.g. ZUBOVIC et al., 1961; DREVER et a/.. 1977; ANDREJKO et al., 1983; SAWVER and GRFFIN, 1983; BENSON and HOLM, 1983; PERRY et al., 1984; HUFFMAN and HUGGINS, 1984). However. as far as we know, only KARNER et al. (1984, 1986) have dealt with the subject in the comprehensive way we consider necessary, in a study of two North Dakota lignite seams. In addition to the methods we used in the present study (described in a thesis by MILLER, 1977), KARNER et al. investigated the topochemistry of elements of use of the electron microprobe. The major deposits of lignites in the North Great Plains province of the United States were formed in large intermontane basins where the seams are generally very thick and often extend laterally over
1311
hundreds or thousands of square kilometres. In contrast. the most prolific occurrences of lignites in the Gulf Coast province are found in near-shore contlnental facies deposited in fluvial, deltaic and sometimes shallow marine environments. Consequenti>. rhc seams in this region arc highly variable in thickness and mostly of small area1 extent. rarely being traccablc for more than a few kilometres. Nearly all the coals in both provinces are Early ‘Tertiary in age. Of the four sets of samples studied in this imestigation. one was from the Powder River Basin in the North (ireat Plains province. and three were from various parts of the Gull‘ province. EXPERIMENTA
The basic information on locatlon. stratlgraph?. methods of collection, etc. is summarized in Table i. ‘The Darco, Texas. sample set was a channel, whereas the other sample sets werr from drill cores. It will be noted that both roof and tloor rock and mineral partings were sampled. as well as the coal seam Extreme care was taken to obtain samples at Intervals immediately adjacent to the seam margins. Mhll sumple. The Wall seam has been StratigraphIcally co-related over a large part of the coal-bearing Tongue Ricer sediments (GLASS, 1976), and was sampled in the north central part ofCampbell County, Wyoming. towards the central edStern margin ofthe Powder River Basin. Its stratigraphic position in relation to other seams, and the profile as sampled by the United States Geological Survey, is shown in Fig. 1. In this. as in the other samples, representative porhons of the sampled intervals were cornposited and placed in the Penn State Coal Sample Bank. These composite samples. for which full andlytical data are available, are indicated b) “PSOC” numbers or letters in the margin of the profiles
All of the important ligmte deposits of the Gulf Coastal Province are found in lower Tertiary rocks exposed in the narrow bands shown in the map in Fig. 2. which outlines the Tertiary shoreline of the Mississippi embayment and shows sampling sites. The general history of the region has been thr~ tilling of a geosynctinal basin by deltaic. fluvial and marine sediments along a southward-dipping axis following the general course of the modern Mississippi River system. The strata arc mostly undeformed. Most of the lignites occur in the nonmarine facies of the Paleocene/Eocene Midway. Wilcox and
C laiborne analogous
Groups deposited 111 rncironmt’nts ~snrcwhal to those of the modern Mississippi deltaic system (FISHER. 1969). The oldest Iignites, found in 4lahama an11 Mississippi in the Upper Paleocene Naheola Formation. ha\c evidently been formed in saline environments. since high sultilr contents are often reported (DANKI. lY73: St-1 I azd Wii& I IAMSON. 1Y77). For the geology at’ fexas coals. WC h %I\! i? (iY74). and FISH~K and McC;owr L I 19671 ~IL(I.CYI sun~p/e The relative11 thlch (7.-3 m i AIIL Ltlmsi:t !Ignite that outcrops and is mined near Darco. I t’u\ hclongs to the Mount Pleasant Fluvial System Mithln :h- WI~CLI: <;roup. As sampled here, there is a lower attrital Irgnitc l~inc forming rhe major portion of the seam, which gmdcs up\\ard into a second section of woody lignite (NICHOI h. i’i”O! 1 !K Jones are usually parted by a clay band of variabic ihlcknes> i&X0 cm). At this site. there is h-18 m of ovcrhurdell. co:1 \icling of shale and sand, and the floor is composed $)Iri (4 rhr peat. lhtern .4luhut~a .wnzpk~~. The ligmtes ofeaslern ilabarna. which are located in the Nanafalia formation ovcrl>ing rhc Clayton limestone, are commonly thick but of limlted lateral cxtcnt. The beds occur as irregular “pods” enclosed by UII~ consolidated sands and silts, and often represent Infillings of peat in abandoned stream channel&. The IO.3 m rhick seam taken from a drill core in Coffee County. Alabama. IS 1) pica1 (1:ig. 5). The samples were taken out at regular 30cm intenali: except near the margins. Both Alabama cores wcrc t~vidrd I>! Shell Mining Ventures, Inc.
Dned samples(N,. 100°C) were crushed to pas5 a iib; ml:. Eve. Representative IO g splits, obtained by ritilinp. W~L lirrthcr crushed to pass a 0. I8 mm qie\,e
Inorganic elements in lignites. II.
1’ -
TJ
”
PSOC 572 DULL COMPACT
-YL
PSOC 573 DULL ___-__---____
1ett.m
along the column jndicots
COAL
COAL
FLOOR: MUDSTONE
CACHE ‘The
PSOC 571 DULLCOMPACTCOAL
u*
146.0
200
1313
sampling
~n+wrols and idmtificotionr.
FIG. 1. Drill core log and lithological profile of the Wall seam (PSOC-570-3).
Methods of analysis Each sample was fractionated by extraction first with ammonium acetate solution and then with 1.0 N HCl, as described previously (MILLER and GIVEN, 1986). Extracts were analyzed by plasma arc emission spectroscopy and atomic absorption flame photometry. The insoluble residue was ashed at 750” and solubilized by fusion with lithium metaborate, and the resulting solution analyzed as above. Mineralogical analyses were made by X-ray diffraction and infrared spectroscopy on the products of low-temperature ashing in an oxygen plasma (MILLER et al., 1979). Pyrite contents were determined by the ASTM standard method (ASTM, 1976).
and Engineering, Inc., Charleston, West Virginia. They were then calculated on the dry, mineral-matter-free basis using the mineral matter contents (Table 2) de-
* 9.5 i-l
-’ .1
T
ROOF, UNCONSOLIDATED
$
UPPER
-
BENCH
BANDED
SAND
(PSOC
WOODY
413)
LIGNITE
G
Basic compositional data The elementary compositions of composite intervals of the coals seams are shown in Table 2. The raw elemental data were obtained by Commercial Testing
MIDDLE
I
2
I
LOWER
BENCH
BENCH
outcrops in southeastern United
(PSOC
414)
415)
FLOOR, CARBONACEOUS
FIG. 2. Paleocene-Eocene States.
(PSOC
CLAY
FIG. 3. Lithological profile of columnar section of the Darco lignite seam (PSOC-4 12).
K, N. M~llrr and P. H. Given
1314
52
ROOF CALCAREOIJS, SAND
-5
-
GLAUCONITIC
**
ROOF
II
DARK
Ilt
LIGNITE PYRITE
_____._.
_-
,B
=
54
GREY
MICACEGUS
-101
FINE
iANG
WITH ABUNDANT NODULES
._.
“CLAYEY” (GRADES
._
LIGNITE INTO PARTING.
-
I CM ----..-.
PYRITE
t.ENY _--_I_.
PYRITE
LENI
CARBONACEOUS CLAY PARTING
=
60
LIGNITE NO VISIBLE
--O’ -I
PYRITF
CM
. FLOCR =
72
GREY
CLAY
WITH
ROOT
CASTS
FIG. 4. Lithological profile of a llgnlte dnll IUX~(X‘H-11 I from Western Alabama.
-- I CMPYRl.llC CLAYEY HICACEGUS SAND the co!.,n:n ,nd,cote thP i-ii.
’ Thenumberr a,ongs,de
,. I.C,a
1’~;.5. Llthological profile of a Ignite dnll iitr,’ 5’ ii--r. ‘.I loom Eastern Alabama.
rived from the ash yield and pynte contents. usmg a modified Parr formula (GIVEN and ~‘j\RznH. 197X). However. this formula is not altogether satisfactory for Iignites; when the content of ash-tbrming c’onstiturnts is high. the calculated mineral matter content can he in error by perhaps l-5’7. Hence there is some uncertainty about the contents of organic elements on a dr->. mineral-matter-free basis. partrcularlv oxygen (csti. mated by difference). It is clear that the eastern Alabama. Darco and Wall samples are of similar organic composition and ranh. while the western Alabama sample is of much lower rank. This latter coal has very high contents of hoth pyritic and organic sulfur as seen in Table 3. The ver;, high content of sulfate in the roof may well indicate that the strata have been extensively weathered. The eastern Alabama seam is also rich 111sulfur, but. surprisingly, nearly all the sulfur is in organic combination. The Darco and Wall seams are of low sulfur contrn ts and evidently formed in fresh water conditions. Corresponding petrographic analyses (0 I TZI IS. 1978) are also shown in Table 3. Liptinite macerals are low in all samples. while macerals of the inertimtc suite range up to 20% or more. The relative amounts of macerals of the huminite group (with differing amounts of cellular preservation) are quite variable between the different seams. but woody tissues in the
Alabama samples evidently dation before burial.
underwent
mucI: degn-
RESUL’I 5
The work is to be discussed here and m a suhsequcrrr paper involved taking many thousands ot’datd points Interpretation of the mass of information L;~~ ire
S.mpl e designation
Wdll
seam PSa: 571 PSOC 572 PSOC 573
u 1,s 2.7.
I.5 2.7 1.9
15.1 7.3 22.9
:‘. : k ‘1.I i i 0 08,.,~ i I.. ,i
‘4.1 73 74 !:
Inorganic elements in lignites. II. Table
3.
Petrographic Seam
and
Sulfur
Analyses
of Major
Sections
InterSample designation Ala., west
Val below roof,
m
Forms of Sulfur (5 as % of coal) pyr. SOI org. tot.
Petrographic
Anal..
vol. %,d.m.m.f. ulm. hdet. lip. in.
A B
0 - 0.9 1.2. 1.4
3.0 1.4
3.2 2.8
4.5 4.8
10.7 8.9
47 57
33 28
2 1
18 14
Ala., A east 8 (SCH. C 67A) D E F
0 0.3 0.8. 2.6 2.6- 4.4 4.4. 6.3 6.3. 8.1 8.1-10.5
0.4 0.5 0.6 0.7 0.8 1.9
0.6 0.2 0.1 0.2 0.2 0.5
5.4 3.7 3.9 3.8 3.9 3.1
6.4 4.4 4.6 4.7 4.9 5.5
24 26 20 23 29 26
53 55 57 52 53 47
6 2 3 3 2 2
17 17 20 22 16 25
Barco Lignite PSOC 413 PSK 414 PSOC 415
0 - 1.9 1.0. 1.9 1.9- 2.8
0.6 0.1 0.1
0.1 0 0
1.2 0.9 0.8
1.9 1.0 0.8
62 62 51
27 18 32
2 2 3
9 18 14
Wall Seam PSOC PSOC PSOC
0 - 1.5 1.5. 2.7 2.7. 2.9
0.5 0.3 0
0 0 0
0.6 0.1 0.3
1.1 0.3 0.3
72 87 91
6 5 5
2 1 2
20 7 2
571 572 573
*Names of maceral suites are designated as fallows: ulm = ulminite + gelinite; hdet = humodetrinite; lip = lipinite suite; in = inertinite suite.
rived from a representative selection of plots of element concentration vusus depth. All of the raw data can be found in the Appendix to a thesis (MILLER, 1977) and a Report based on this thesis (MILLER and GIVEN, 1978).
Mineral distributions The yields of ash from acid-insoluble fractions show considerable variations with depth in the various cores. In the Wall, insoluble inorganic matter (as measured by ash yield) tends to decrease fairly steadily with depth from the upper margin of the seam, while in the Darco and Alabama seams it tends to be highest near the upper and lower margins. In contrast, the total content of extractable ions (as measured by the difference between ash yield from the whole coal and from the acidinsoluble part) does not vary much with depth in any of the seams, and constitutes as much as 50-80% of total inorganics, a notably high range. Of course, different phenomena govern the contents of the two kinds of inorganic matter. The input ofboth siliceous particles and cations in solution is adventitious, depending on source and ease of transport, which can change during the accumulation of one peat bed. However, the amount of particulate matter retained depends also on the filtering ability of the peat, not having a defined upper limit, whereas the uptake of cations is limited by chemical equilibria. These points perhaps account for the differing trends of the two kinds of inorganic matter. The changes in pyrite content are illustrated in Fig. 6. Pyrite content is low or very low in the Wall and Darco seams, except for short intervals in the middle of the seams. Large nodular pyrite accounts for 6.5, 39, and 24% of the sediment in the roof section of the Wall, the parting (SS) and the top section of the coal (T 1), respectively; a post-burial origin seems likely, the greater part of the seam evidently having been deposited in fresh water zones of the fluvial/deltaic environment.
1315
In the west Alabama seam, the pyrite content is high and very variable in the upper benches, but quite low in the lower 50-60% of the seam; organic S is about 4.7% throughout. On the other hand, in the east Alabama seam, the content rises from about 1.2% of dry sediment in the upper half of the seam to 1.5-8.5% in the lower half. It should be noted that the organic sulfur content of this seam averaged 5.4% of dry coal in the top 30 cm and was close to 3.8% throughout the rest of the 10 metre profile. Evidently the original peats were sites of intense growth of sulfate-reducing bacteria and variable iron content, as is now the case in the coastal mangrove swamps of the Florida Everglades (GIVEN and MILLER, 1985). All of the profiles show fairly sharp maxima, but these do not coincide with the presence of mineral partings. The sediments in the Porter’s Creek Formation, which probably constituted coastal highlands when the west Alabama seam was accumulating, are known to be rich in siderite and limonite. Leaching of either or both of these minerals from the Porter Creek sediments could have been the source of the pyritic iron in this lignite and its roof rock. A number of minerals were identified by X-ray diffraction in the roof and floor rocks, the partings and in the low temperature ash from the lignites, and the results present some points of interest. In the roof and floor surrounding the Wall seam, but not the coal itself, alkali feldspar was found in addition to other silicates. For a feldspar to reach a sediment more or less intact, rapid transport from the source rock and fairly rapid burial are required. These conditions could well have been met in the intermontane Powder River Basin where the Wall seam occurs. That feldspar was nevertheless not found in the coal suggests that there was some decomposition mechanism operating with the peat; it is known that organic acids, particularly those with chelating ability, can leach ions out of silicate minerals (EHRLICH, 1981). Much more kaolinite was found in the coal of the Darco seam than in the roof or floor. The roof is mostly quartz, and the floor principally a mixture of quartz, kaolinite and illite, while a variety of clays was found in the parting. The relative abundance of kaolinite in the lignite suggests that it was generated in situ by alteration of other clays, through leaching of cations by organic acids. Other evidence supporting the same interpretation will be presented later. Some minor minerals were identified in a heavy gravity fraction of the roof rock (s.g. 2.85): rutile. zircon. tourmaline. hematite and ilmenite. The minerals in roof, lignite and floor of the east Alabama seam were principally quartz and kaolinite, but quartz was much more dominating in roof and floor and was there accompanied by muscovite (a potassium aluminosilicate). The minerals in the coal tend to be concentrated near the upper and lower margins of the seam. Intense low-angle x-ray scattering by the low temperature ash from the west Alabama lignite made
K N. Miller and P. H. Given
FIG.6. Profiles of’ pyrite contents tn tour seams’ 131 I)arco. th) West Alabama. (c) East Alahuma. id! ‘A .!i,
identification of clays difficult, but the parting and floor were shown to consist mostly of various clays. The roof is quite different; it contains abundant oval grain> of glauconite, aragonitic shells, many forms of pyrite. quartz grains, and minor amounis of calcite and ilk. Glauconite, an iron-potassium aluminosilicate. :s thought to be formed in shallow marine waters under reducing conditions (CLOUD. 1955: KKUMBEIN and SLOSS, 1963). It has a high magnetic susceptibility. and this was used to isolate some grains of the mineral and confirm its identity by SEM with stimulated x-m! emission.
The elements that were to an appreciable extent es-. tracted by ammonium acetate solution were <‘a. Mp. Sr. Ba, Na. K and Mn. The ranges of concentration found, the mean values and the covariance (ratio ot standard deviation to the mean value, expressed as ~1 percentage) are shown in Table 4. In general, the concentrations of the elements named tended to van in
Sample
Darca
I?
Range(5llii. 6870 Mean
6400
c0v.z 14
7
Range MeaIl
7910. '10300 a700
express
in ppm
CO”.%
l
**
rangeand
AMMONIW
ACEln!E
iOLuBU
No. of
iignite Wall seam
a parallel manner m any one seam. Ho\lc\:-r ti;~:are appreciable differences in concentmk.)l~ ;tnri 1:’ the Ca/Mg ratio. between seams. (Figs. 7 li ;iit)\s cx amples.) Ca is particularly high (mean val~i i II, i>j drl coal) in the east Alabama heam, WI~I-Iiii>:. ii~i&ci: tluctuation with depth. Overall, the dilkrencts hctwczn seams are no doubt due to local differences ;;I g;ourtii or surface water composition. While clays do contain exchangeable iaucjns. !IIL concentrations of all ammonium-acetate-exli’act~tisl~ cations drop drastically as one passes through rhc margin of the lignite seam into the roof or floor !ocL. and in partings, confirming that the catlons arc pnnclpal!:, attached to the organic matter. as carboxyla~c salts. Ii will be seen that there is some fluctuation ~$conceri tration with depth. but uniess there is a parung:. j ~iiut_-
meant
4
ii,-
.%+ is- 0, is ,i,: 4" I , 1. ,~ 2’
IDlO1340 1250
i,>V L4D ;'i5
33c 240
27'8 230
2370. 3220 2720
Ill& 260 70;
3X750 41 0
10301180
7
:< 17
11
of dry
‘1
coal
,uc150
31;. hi
i 1In ,& i,‘T 3 ‘I
0
90
180
270 360
PPMOFDRYSEDIMENT
0
40
81
IZI! 16" OK
PPMOFDRYSEDIMENT
(/II;L itiJ Sd 3 ',
=':. OFDfi* SEDIMENT
FIG;.7. Distribution of some Ion-rxchangeahic ihc Darco Eeam
&wt.ni\
t!!
Inorganic elements in lignites. II. AMMONIUMACETATESOLUBLE CALCIUM
1317
Table 5.
MAGNESIUM
SOoIUm
Distribution of the Major Alkali and Alkaline Earth Metals and Maganese in the Chemical Fractions
Sample designation
143.0
Amman. acct. ttC1 sol. sol.
Ala.,
Ca
92
7
west (7)*
Mg Na K Sr Ba
79 79 5 91 **
32 2 7 **
Mn
68
25
Ala., east
Ca Mg
91 a8
8
(x)*
; Sr Ba Mn
;; 95 ** 42
: ** 47
Ca Mg Na K ;;
35 88 77
10 4 3
Mn
A? ** 58
; *t 38
Ca Mg
77 90
18 8
143.5-
144.0
144.5-
145& Darco Lignite
145.5
(17)* ‘14b.m + 0.050.300.55 0.601.050.00 0.090.1s0.27 0.360.000.040.080.120.16 4OFDRY
SEDIMENT
%OF
DRY SEDIMENT
%OFDRY
SEDIMENT
FIG. 8. Distribution of some ion-exchangeable elements in the Wall seam.
Wall seam (14)*
* do not vary much (except for the west Alabama core, not shown). The ratio, Ca/Mg, is appreciably different in the coal seams compared to the adjacent strata. Thus is is 5-6 and 1.8 in the roof rocks of the east Alabama and Darco seam respectively, and 9 and 2.9 within the seams. Throughout all seams, the ratio of Ca/Mg is higher in the coal horizon. This finding is consistent with the Ca/Mg adsorption results obtained on the North Dakota lignite (discussed previously; MILLER and GIVEN, 1986), where it was shown that Ca was enriched in a lignite relative to the concentrations in a mixed Ca + Mg solution used for ion exchange in a laboratory experiment.
AMMONIUMACETATESOLUBLE CALCIUM
MAGNESIUM
4
STRONTIUM
f
0.0 0.7 1.4 2.1 2.8 0.000.04 0.080.12 0.16 0 5 OFDRY
SEDIMENT
XOFDRYSEDIMENT
60
%OFDRY
FIG. 9. Distribution of some ion-exchangeable the Eastern Alabama seam.
,
120 180 240 SEDIMENT
elements
in
**
NK" Sr Ba Mn
;: 54 60 49
Acidinsol. Residue 1 ;; 93 1 ** 7 :
z
: 14 18 48
s: 0 ** 11 4 2: a4 5 ** 4 5 ; 27 32 22 3
number of sections used in the analyses
acid-soluble data unavailable for calculating totals
The coal insoluble in ammonium acetate was extracted with 1 N HCl, which will dissolve soluble or reactive salts (carbonates, some sulfides and sulfates) and certain iron oxides (limonite, hematite, goethite), and will solubilize metals from many coordination complexes, including some chelates. Mean relative concentrations of seven elements in the various fractions of the four lignites are shown in Table 5. Most of the Ca, Mg, Sr and Na in the coals is indeed ion exchangeable. A substantial part of the Mn, and of the Ba in the one case for which data are available, are also extracted by ammonium acetate. Only the Wall seam has much of its K in ion exchangeable form; this coal also has less of its Sr exchangeable than the others. There appear to be small amounts of calcite or other carbonate in the seams, but Ca really calls for no further discussion. There is some acid-insoluble Na in the east Alabama and Wall seams. The sodium in the associated inorganic horizons is almost entirely insolubie in acid and the content is low (ranging from 0.009 to 0.56%). The alkali feldspar in the roof and floor of the Wall seam will hold some Na, but otherwise, and in the coals, the insoluble Na will be in clays. The profiles of ion exchangeable Sr and Ba in the Wall seam (Fig. 10) are similar to those of Ca and Mg in the same seam (Fig. 8), indicating the same mode of occurrence. However, significant concentrations of these elements were found also in the acid-soluble and acid-insoluble parts of the seam, with profiles very dissimilar to those of the elements soluble in ammonium acetate. This appears to be a general phenomenon for all seams. The acid-soluble ions are no doubt present as carbon-
ates (barium carbonate was found in the Hagel scan in North Dakota; MILLER and ‘&\,I-Y. 1986). .Icidinsoluble Ba is likely to be in harite. possibl) SI ;I\ sulfate as well: no acid-soluble sulfates were tbunJ The acid-insoluble metals tend in the Wall seam to hc confined to the lower part of the seam t;>r come uncx plained reason. A typical example of the distribution of Mn 11~the fractions is seen in Fig. 1I in 311timi- seams. tlli amounts ofMn extractable by ammonium acetate and by HCI are roughly equal. and between them the twcl fractions account for 90% or more of the element pres ent in the coals (Table y). In the partings, roof and floor rocks, however. both these fractions are vq small, and acid-insoluble Mn dominates (presumabl! as MnO,). The similarity of the distributions of Mn IG the two soluble fractions suggests that the acid-soluble part is coordinated to organic matter. ;I\ ;t transition metal. Mn does form stable orgamc complexes (i’ij; TONand WILKINSON. 1980). However. S\\“\r’\it ( 19x6~. while agreeing that some MI] in Australian brown coals is present as carboxylatc salts. believes that the PI inclpai inorganic forms ofthe element in coals are as irnpuritic\ in calcite, siderite, and sphalerite. all ofwhizh arc ac~tl soluble. In order to discuss acid-insoluble K and Mg. some remarks about Si and Al must bc ma& as well. 45 expected from the mineralogical analyses. all ot the SI and much of the Al are in the acid-insoluble lYact~an\ where they are combined as silicates. I‘hc profiles of all four elements are more or less parallel, and there are correlations of acid-insoluble K and Mg with the content of clays, confirming the obvious suggestion that the metals are tightly bound in clay minerals. as f’o~ example, the K in illite and mixed-layer clays. An cxample of a set of profiles is shown in Fig. 12. where the markedly parallel variations can be readily seen. The ratio, K/Al (acid-insolubles), is also shown In Fig. 12 for the Darco seam. This ratio serves as a useful index of the type of clay present. It is always higher in the primarily inorganic horizons. but very low within
V>‘I.? I !li the roof. 0.2U in the parting and 0. I6 in the liuor rock, but never more than 0.07 in lhl: coal. in !II;, i.1~3 .tiabama seam acid-insoluble Al is i .Y- ~o!‘tl~ wcl~nwr~: III both roof and coal. indicating a near-l! *_onstaIii c.ontent of clay, while the amount of k 111thcsc &I> minerals is quite different. the K/Ai ram.’ cfroppln$! from about 0.5 in the roof to 0.05 in the co‘i‘ it seett~s reasonable to suppose that the delrn:il ITI~!I crals deposited in and immediately over a SG~I wuu! t)rganismz would have been present. 1x11 tot III lirc primaril> inorganic zones. Thus e.xtcnsi\e IUS of h: and ti)rmation I>[authigenic kaulinitz took pijc,c. i lx phenomenon w_vn~ to be quite general 111:ill of’ tlrt* ,eams studie~i the coal. 3’hus. In the Darco scam. the ratIt
AMMONIUMACETATE SOLURLF AC!!: V;’ i’y’ E
li, :I! INK)L!,B, t
4460 ii u I8 36 54 12 0 ?? is 0 I8 36 54 72 0 PPM OF DRY SEDIMEKT PPM OF DRY SEDIMENT PPY W DRY SEDMEN!
Inorganic elements in lignites. II.
9 OF DRY SEDIMENT
“. OF DRY SEDIMENT
5 OF DRY SEDIYEHT
1319
WY OF DRY SEDIYEN,
5 OF DRY SEDMEKT
FIG.12. Distribution of some acid-insoluble elements in the Darco lignite seam.
Acid-soluble Al, Ti and Fe The change in the degree of acid-extractability of Al, Ti and Fe between the organic-rich and the inorganic-rich horizons of all four seams provides a great deal of evidence that the form and distribution of these elements is markedly affected by interactions with organic and biological processes in the coal bed in a manner not appreciated hitherto. The interpretation is not identical for all three elements, but the important phenomenon is observed with all, that the fraction by weight of Al, Ti and Fe that is extractable by acid is always much higher in the organic horizons than the inorganic, regardless of the absolute amount of the elements present. As examples, Fig. 13 shows the profiles of acid-soluble Al in two seams. Thus the concentrations in roof and floor are about 0.0 1 and 0.06% respectively in the east Alabama seam, and 0.12-0.25% in the organic layers. In fact, acid-soluble Al can account for up to 50% of the Al in a seam. In no case is there a correlation between acid-soluble and insoluble Al, contrary to what one might have expected if some Al were leached from clays during extraction with HCl. It therefore seems likely that acid-soluble Al is complexed by organic matter (how this might happen is discussed below). In previous work with a North Dakota lignite (MILLER and GIVEN, 1986) the greater part of the acid-soluble Al was found in the fractions of lower specific gravity, thus excluding the possibility that the extractable Al is in the form of hydrated oxides, and hence the same conclusion was drawn. The breakdown of detrital clays by organic matter, discussed above in connection with K, could provide a source of Al in soluble or colloidal form. The complexation of Al with humic acids has been reported by LAKATOS et al. ( 1972). A word of explanation is perhaps needed for the inference that some metals are present in lignites as coordination complexes with organic ligands. In the nature of things, one cannot isolate and identify coordination complexes in coals. One can only draw in-
ferences from the behavior of elements in physical or chemical fractionation and the known chemistry of the element. Three types of compound known to form many chelate coordination complexes are salicylic acid (I), I-hydroxy-9, IO-anthraquinone (II), and catechol (III).
COOH
0 H\O
&OH q$
OH
&OH
0 I
II
III
A proton is lost as a metal ion takes its place, bridging the two functional groups. We envisage such structures forming part of the lignite macromolecule. Complexes of II with Al+++ and Fe+++ among others, are known to be easily formed and particularly stable. Examples of Ti distributions are shown in Fig. 14 for the east Alabama and Wall seams. As can be seen, the acid-extractability of Ti in the roof, floor and partings constituting the inorganic horizons of the seam is very low, while in the organic horizons it is generally much higher, tending to peak near the margins. Considering all four sites, l- 10% of Ti present is extractable by acid from the inorganic horizons, but 30-60% in the coals. None of the common Ti minerals is soluble in dilute mineral acid. Also, leaching of Ti from clays must again be ruled out as a major factor. In all cases, the acid-insoluble titanium remains high in the roof and floor, but tends to be low in the coal. We infer that the acid-soluble Ti is organically complexed, and that the acid-insoluble in the inorganic rocks is present in mineral forms. The same may be true of the insoluble Ti in the coals, but not necessarily. It should be noted that KARNER et al. (1986) do not report acid soluble Ti in the two North Dakota seams they examined.
K. N. Miller and P. H. Given
133)
ii i1
i
64
-1-7
0.00
0.06 % OF DRY
0.08
0.12
0 16
$---t---. 000
0 06
0.12
0.1.9
0 24
% OF DRY SEDIMENT
SEDIMENT
FIG. 13. Distribution of‘acld-soluble aluminum rn the Darcc~ lignite and eastern .4labama seam<
In our study of the North Dakota lignite MILI.I K and GIVEN, 1986). it was found that most of the acidsoluble Ti occurred in the fractions of lower specific gravity, but so also did an appreciable (though lowerj proportion of the acid-insoluble. We therefore postulated that some of the acid-insoluble Ti was organicall> bound. citing the work of ESKE.NKC~ ( I Y72), who studied the adsorption of Ti on peat humlc acids and coals in the pH range 0.X-2.0. and found that oni\
part was desorbed in HCI of p1-l I .4. In lhc PI-esent work we did not perform gravity fractionations and SC> have no evidence to show whether some of !bc XXinsoluble Ti in the lignites is organically h& 4 plausible hypothesis explaining hog +r’gan~ complexation of Ti could come about can be proposed. The mineral ilmenite (FeTQ\ often OCCUI-L ii: sand:. and it was in fact detected in a heavy grabi~; !i’actthr. of. the sandy roof of the Darco scam. NW I[ 15Lnw F that, in mildly acidic and reducing conditi
DISCUSSION
AND
CON(‘I.1 !-dONh
.4s was stated earlier, the prtncipal object1 Ws of’thrs study were to determine how major, minor V.c t;a\~:’
..
-,
q
,631, 18Lr 364
54”
PPM OF DRY SEDIMENT A
EGTERN
72”
ooooii
022
033
” OF DRY SEDIMENT nLPlBn~n
IE&M
03L
r
0
110
220
-
330
7
7
440
00””
PPM OF DRY SEDIMENT u
Fii; 14. i>istribution t>ftitanium In two hgnites
-
I”
,,A
/’ 4:‘
. OF DR? SEDIMEN1 WALL
StAa
I’
Inorganic elements in lignites. II.
it can be said now that this also is much influenced by the fact that we are dealing with an organic sediment. The phenomena discussed seem to be quite general for peats and lignites, though the magnitude of effects varies from site to site. Vegetational changes within and between peat-forming ecosystems are indeed significant and will, no doubt, in part determine the inorganic signature remaining within the preserved organic matter. However, the pH of the connate water in peats is variable and will obviously have a number of effects on solution equilibria and hence on the fate of many of the more mobile elements. Ionic strengh, which varies greatly between different peats, likewise has a considerable effect on rates of diffusion of ions and even on rates of bulk flow of water (GIVEN and DICKINSON,1975). Some water in peat is “trapped” and not available at all for diffusion. The amount and nature of the specific functional groups will also play a considerable role in what inorganic matter is found there. In fact, peats are very complex physicochemical and colloidal systems (GIVEN and DICKINSON, 1975).
DPlRCO SEAM
i
I
I
I
0.140.200.420.56 X OF DRY SEDIYEHT
,
I c 0.36 0.54 D.00 0.18 X OF DRY SEDIMENT
1321
I
072
FIG. 15. Distribution of acid-soluble iron in the Wall and Darco lignite seams.
fractionated a number of the major and minor inorganic elements in lignites on the basis of solubility in various reagents, and have successfully related these solubility data to mineralogical observations and seam profile analyses (of both organic and inorganic zones) to draw inferences on the predominance of organicinorganic interactions. Though the results are not unequivocal in all cases, the methods used have permitted a fair degree of success in achieving the objectives. Some of the interactions inferred have not been discussed previously. Our identifications of ion-exchangeable elements are similar to those of KARNER et al. (1986), though we find greater differences between seams. Our observations and conclusions relating to Ti are different. The most notable observation here and in Part I (MILLER and GIVEN, 1986) is the large extent to which the organic sediment determines what inorganic matter it associates with. Thus substantial proportions of the ash-forming constituents, such as Ca, Mg and Na, are present as carboxylate salts. Some Mn, Al, Ti and Fe are most probably held as coordination complexes by the organic matter. The generation of pyrite stems from bacterial respiration, which requires rather specific organic compounds as fuels, and these are probably formed by fermentative processes of other organisms, such as Clostridium sp. (BELYAEVet al., 1981). Carbonates are formed in situ largely as a product of aerobic respiration of roots and microorganisms. Organic acids in the sediment cause significant alteration of clay minerals and degradation of accessory minerals like ilmenite and feldspars. The distribution of trace elements will be discussed in a subsequent paper, but
Lignites no doubt will inherit most of the results of phenomena in peats, but are themselves subject to alteration by post-depositional processes involving the movement of ground water through the seams. The relative contributions of peat and post-depositional phenomena, particularly with regard to highly mobile elements, is difficult to define though this study has shed some light on the importance of both. The study discussed here and earlier (MILLER and GIVEN, 1986) has been almost entirely concerned with the intrinsic nrocesses in peats and lignites, and these are of course of great importance in determing what suite of inorganic materials a lignite will eventually contain. Other important determining factors are the extrinsic ones-that is, the nature of the rocks being eroded around the coal-forming basin and the local hydrology as it affects transport of mineral grains and ions in solution during and after peat formation. These additional factors require a separate study, which we have not made, though such a study would be an important complement in seeking to understand the inorganic geochemistry of coal-forming systems. KARNERet al. (1986) and several other authors have reported that some major and trace elements and/or minerals tend to be concentrated near the margins of seams. There seem to be disagreements on which elements and on the underlying reasons. We shall discuss this matter with our own data in Part III of this series of papers.
Acknowledgements-This project was supported in part by the U.S. Dept. of Energy under Contracts E(49-18)-2030 and E(49- 18)-2494. The coal samples and their basic characteristics were drawn from the Penn State Coal Sample and Data Base, assembled under the direction of Dr. William Spackman. The authors are indebted to Mr. Norman Suhr and Ms. Marion Briscoe for analyses by emission spectroscopy.
Editorial handling: J. W. de Leeuw
R N. Miller an
1.321 REFERENCES
ASTM ( 1976) Annuul Book oJ’.1STM Slundmds. Part ‘6 Gaseous Fuels. Coals and Coke. Standard D 2497. KARNER F. R.. I~ENSONS. A.. S( !IORIXI Ii. ki. ~1x2 K and M. R. WALTER),pp. 235-242. Springer-Verlag. and their metal complexes. In I’!;,: ~‘Oii~i/!!:lir,‘i;ri!i:1’!,i! BENSON S. A. and HOLM P. L. (1983) Comparison of inorPro/ (‘on~rf>\s 4, pp. 341-353. Otaniemi. I-inlsn\i ganics in three low-rank coals. Preprints. Div. Fuel Chem I YND L.. F:. i 1960) Study of the mechanism XXI rdtc ill I! Amer. Chem. Sot. 28[2J, 234-2.39 menite weathering. ,4mcr In\i tliwrul .\l,?tii! i 8:; ! *i:vt BROCKT. (1984) Biology of’~~i~rc,or~~~~:r,s~~~\,4th edn. Prentlcc217, 311-31x. Hall. Englewood Cliffs. New Jerse:! Mii L~K R. N. (1977) A geochemlcai stud! <)I!i;:: 1rlulgan1e CAROI D. (1960) llmenite alteration under reducing condoconstituents in some low-rank t,tlals. Ph D :%.~I+.P?n? tions in unconsolidated sediments E~,o?i (&o/ 55. 6 18 ;lh&+iignlic, distribution of As. Ba, Cd. Ca, Hg. Mo, Pb and I! associated (;cz,,c,/lirn C‘ovmochim. A&I 50. ‘033--2047 with the Wyodak coal seam. Powder River Basin. Wyoming MIL.L.ERR. N.. YARZABR. F, and GIVEN ?. H 1I~~‘%Ii>t, c’nntrih. tieo1og.v 15, No. 7. 93--IO I. termination of mineral matter contenTs (11i i::ai<1,s tgj,*EHRLICHH. L. (1981) Geornlcrohroio)~~,.pp. ‘!X-79. ! 2% I i(: temperature ashing. Fuel 58, J-- IO Marcel Dekker, New York. UI(‘tjOLs D. J. (1970) Palynolog) in I-elatloll to ~iq~os~ti~ln,:i ESKENAZYCR. (1972) Adsorptmn of tltamum on peat and environments of lignites in the Wilcox Group ! I Iutiar) ) coals. Fuel 51, 221-223. in Texas. Ph.D. thesis, Pennsylvania State Uni\!: .4.s.soc.Geol. So,. Truns. 19, 239-26 I (‘hcwisrr~~ o/‘l.m,-runk Coui,~ (ed. H. FI. %‘HOPIKi ) ‘\I:i.‘: FISHERW. L. and MCGOWENJ. H. ( 1967) DeposItional sqs(‘hem. Sot. Symp. Ser. 264. 1-- I4 terns in the Wilcox Group of Texas and their relationship RENION J. J. ( 1982) Mineral matter in coal. In i t ‘u: ii,‘licirr;/ to the occurrence of oil and gas. Gz& (‘<‘o\/ .-1~.to(’ fIc,oi ted. R. ,A. MFYERS),pp. 783--I! 36. ,4cademli Prc\\ ‘x\‘,* SK Trans. 17, 105-12.5. \ ork. GIVEN P. H. and DIC‘KINSON C. t-I. ( 1975) Hmchemlstry dnd SAWYERR. K. and GRIFF~VG. M. [ 198.1) 1hc auc LLn.: microbiology of peats. In SOI/ Biochrmislry 3 (eds. E. ,\ origin of the mineralogy of the northern Flonda 1rergladc? PAUL and A. D. MCLAREN),pp I L-2 I? Marcel Dekke:. In Pm If ‘orkshop. Mineru! .llgf/cr itr l’mtf\(cl:‘. R. Ri 5 New York. MOND and M. J. ANDREJKO).pp. 18% 19s i “\ ~1:m 1. GIVEN P. H. and MILLERR. N. ( 1985)Distribution offorms Nat. Lab. Rept. LA-9907-OBES. of sulfur in peats from saline environments in the Florida Stat L1. M. and WILLIAMSOUD. K. ( iY7.7) occilircrl~~e .iliCl Everglades. Int. J. Coul Geol. 5, 397-409. characteristics of Midway and Wilcox hgnites 1n Mississippi GIVENP. H. and YARZABR. F. (1978) Analysis ofthe organic and Alabama. In Cieolog!,q/ :l/rcwc~c~~ FXT~I. i<:wr~rr~~‘i r/r substance of coals: problems posed by the presence of min1hc Soulh Cenrru/ l’.S. (ed. M. D. C\MPHtl I_ :q’ I(;’ eral matter. In Anaiyticol Method~ylvliv C’oul and (‘ON/ Prod 177.Houston Geological Sot Houston. Ic:<:l\ UCIS2 (ed. C. KARR), pp. 3-4 1, i\cademic Press. New York 9~i ki? L. G. and MARTELLA I.. (1~7I J Stabrhi\ ~w~akmi~ GLASSG. B. (1976) Update on the Powder River coal basin of metal-ion complexes. (‘l~,r>i .Soi, 'i.iJfitltVi ')]Q'! Pi!!" In Geology and Energ), Resourcc~.\ of’the P(wdw River (cd. 3. Suppl. No. I, 4848. R. B. LAUDON).pp. 209-221. 28th Annual Field ConfcrSW~INED. J. (1986) Inorganic manganese i:) <‘,$.!1I-:+(?bi;. ence Guidebook, Wyoming Geological Assoc. 1622-1623. GUTZLERR. (1978) Petrology and deposltional environment\ T’EMPLE A. K. (1966) AIteratIon clt’llrnenite. c,‘.:,‘*i rtis,l bE of Tertiary lignite in Alabama. Ph.D. thesis. Pennsylvama 695-7 13. State University, 443~. Zueovlc P.. S~ADNICHEN~O: .ind 9tL.w : i li, i iw! I HUFFMANG. P. and HUXZINS !-. E. (I 984) Analysis of the Geochemistry of minor elements in coals ot the Northeril inorganic constituents in low-rank coals. In The Chenzu/q Great Plains Coal Province. I. C Ciao/S:cr-1. li!f!! ! f J_ ! of Low-rank Coals (ed. H. H. SCHOBERT).4mer. Chem ‘7p. Sot. Symp. Ser. 264. l59- 174.
Gw
+ .oO
The association of major, minor and trace inorganic elements with lignites. II. Minerals, and major and minor element profiles, in four seams ROBERT N. MILLER* and PETER H. GIVEN** College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, PA. 16802, U.S.A (Received October 16, 1985; accepted in revised,form February 16, 1987) Abstract-Inorganic elements in lignites may be present in carboxylate salts and organic coordination complexes as well as in discrete mineral phases. Moreover, organic matter can alter mineral phases in peats. The objectives of this study are to investigate how a number of important elements are held in some typical lignites and hence to document organic/inorganic interactions of various kinds. Accordingly, detailed profiles of the distributiop of IO major and minor elements through four lignite seams have been made, including roof and floor rocks and any mineral partings. Elements extractable by 1.0 N ammonium acetate and by 1.O N HCl and in the insoluble residue have been analyzed by plasma arc emission and atomic absorption spectrometry. Mineral phases were analyzed by x-ray diffraction and infrared spectrometry. Substanial proportions (40-90%) of the Ca, Mg, Sr, Ba, Na and Mn were extractable from the organic zones, but not the inorganic, by ammonium acetate, and were inferred to have been present in the organic matter as carboxylate salts. Much of the K and some Mg were present in clay minerals, but K-containing clays were depleted in the coal layers relative to roof and floor, probably because organic acids in the peat had leached the K out of illite, thus altering the clays. Part of the Mn was in the organic matter either complexed or dispersed as acid-soluble mineral (e.g. MnS, MnCO,). Acid-soluble Al and Ti were found to be quite abundant in the organic layers but not in the roof and floor; they probably are complexed with the organic matter. The principal conclusion is that the organic matter in peat and lignites has a profound influence, in many ways, on the suite of inorganic materials with which it becomes associated. INTRODUCTION COAL ISA SEDIMENTARYrock containing both organic and inorganic materials. In the case of coals of high volatile bituminous and higher rank the inorganic materials are largely discrete mineral phases. Peats and low rank coals contain 1-4 meg/g of carboxylic acid groups, which can bind cations in salt form. These and other functional groups, can also hold cations as coordination complexes. Such complexes, particularly if they involve chelation, can have (negative) free energies of formation of many tens of kilocalories per mole (SILL~N and MARTELL, 197 l), whereas ion exchange processes involve the release of only 0.2-2 kcal./mole (GIVEN and DICKINSON, 1975 and references therein). Cations held in these ways can constitute 50% or more of the ash-forming constituents in a lignite. It is known that organic materials can alter certain detrital mineral grains (e.g. ilmenite, illite: CAROL, 1960; TEMPLE, 1966; EHRLICH, 1981; BAILEY and KOSTENS, 1983). The presence of various microorganisms can promote the deposition of such authigenic minerals as pyrite, carbonates and opaline silica (BROCK, 1984; EHRLICH, 1981; ANDREJKO et al., 1983). These various processes will have occurred at the peat stage, but the effects will remain after coalification to high rank. Thus the distribution of inorganic matter in coals presents some interesting problems on the borderline between organic and inorganic geochemistry, and these problems will be especially chal-
Present addresses: * Air Products& Chemicals,Inc., P.O. Box 538, Allentown, PA 18105. ** Calda Cottage, 5 High
Street, South Woodchester, Stroud, Glos., GL5 5EL, England.
lenging when peats and low rank coals are considered, since with them a wider variety of phenomena will be important, involving interaction between organic and inorganic substances. It is the purpose of the series of papers of which this is the second to address these problems as they arise in the study of five lignites from various basins in North America. We have already published an account of our procedures and their application to a column sample of a North Dakota lignite seam (MILLER and GIVEN, 1986). We offer, here, data for four lignites from other localities, which were available as drill cores including the roof and floor rocks. Much published work on inorganics in coals (reviewed by MILLER and GIVEN, 1986; RENTON, 1982) has been restricted to samples of high volatile bituminous and higher rank; the nature of any association with organic matter is generally not discussed. A number of recent publications do deal with certain aspects of the occurrence of inorganic matter in peats and lignites (e.g. ZUBOVIC et al., 1961; DREVER et a/.. 1977; ANDREJKO et al., 1983; SAWVER and GRFFIN, 1983; BENSON and HOLM, 1983; PERRY et al., 1984; HUFFMAN and HUGGINS, 1984). However. as far as we know, only KARNER et al. (1984, 1986) have dealt with the subject in the comprehensive way we consider necessary, in a study of two North Dakota lignite seams. In addition to the methods we used in the present study (described in a thesis by MILLER, 1977), KARNER et al. investigated the topochemistry of elements of use of the electron microprobe. The major deposits of lignites in the North Great Plains province of the United States were formed in large intermontane basins where the seams are generally very thick and often extend laterally over
1311
hundreds or thousands of square kilometres. In contrast. the most prolific occurrences of lignites in the Gulf Coast province are found in near-shore contlnental facies deposited in fluvial, deltaic and sometimes shallow marine environments. Consequenti>. rhc seams in this region arc highly variable in thickness and mostly of small area1 extent. rarely being traccablc for more than a few kilometres. Nearly all the coals in both provinces are Early ‘Tertiary in age. Of the four sets of samples studied in this imestigation. one was from the Powder River Basin in the North (ireat Plains province. and three were from various parts of the Gull‘ province. EXPERIMENTA
The basic information on locatlon. stratlgraph?. methods of collection, etc. is summarized in Table i. ‘The Darco, Texas. sample set was a channel, whereas the other sample sets werr from drill cores. It will be noted that both roof and tloor rock and mineral partings were sampled. as well as the coal seam Extreme care was taken to obtain samples at Intervals immediately adjacent to the seam margins. Mhll sumple. The Wall seam has been StratigraphIcally co-related over a large part of the coal-bearing Tongue Ricer sediments (GLASS, 1976), and was sampled in the north central part ofCampbell County, Wyoming. towards the central edStern margin ofthe Powder River Basin. Its stratigraphic position in relation to other seams, and the profile as sampled by the United States Geological Survey, is shown in Fig. 1. In this. as in the other samples, representative porhons of the sampled intervals were cornposited and placed in the Penn State Coal Sample Bank. These composite samples. for which full andlytical data are available, are indicated b) “PSOC” numbers or letters in the margin of the profiles
All of the important ligmte deposits of the Gulf Coastal Province are found in lower Tertiary rocks exposed in the narrow bands shown in the map in Fig. 2. which outlines the Tertiary shoreline of the Mississippi embayment and shows sampling sites. The general history of the region has been thr~ tilling of a geosynctinal basin by deltaic. fluvial and marine sediments along a southward-dipping axis following the general course of the modern Mississippi River system. The strata arc mostly undeformed. Most of the lignites occur in the nonmarine facies of the Paleocene/Eocene Midway. Wilcox and
C laiborne analogous
Groups deposited 111 rncironmt’nts ~snrcwhal to those of the modern Mississippi deltaic system (FISHER. 1969). The oldest Iignites, found in 4lahama an11 Mississippi in the Upper Paleocene Naheola Formation. ha\c evidently been formed in saline environments. since high sultilr contents are often reported (DANKI. lY73: St-1 I azd Wii& I IAMSON. 1Y77). For the geology at’ fexas coals. WC h %I\! i? (iY74). and FISH~K and McC;owr L I 19671 ~IL(I.CYI sun~p/e The relative11 thlch (7.-3 m i AIIL Ltlmsi:t !Ignite that outcrops and is mined near Darco. I t’u\ hclongs to the Mount Pleasant Fluvial System Mithln :h- WI~CLI: <;roup. As sampled here, there is a lower attrital Irgnitc l~inc forming rhe major portion of the seam, which gmdcs up\\ard into a second section of woody lignite (NICHOI h. i’i”O! 1 !K Jones are usually parted by a clay band of variabic ihlcknes> i&X0 cm). At this site. there is h-18 m of ovcrhurdell. co:1 \icling of shale and sand, and the floor is composed $)I
Dned samples(N,. 100°C) were crushed to pas5 a iib; ml:. Eve. Representative IO g splits, obtained by ritilinp. W~L lirrthcr crushed to pass a 0. I8 mm qie\,e
Inorganic elements in lignites. II.
1’ -
TJ
”
PSOC 572 DULL COMPACT
-YL
PSOC 573 DULL ___-__---____
1ett.m
along the column jndicots
COAL
COAL
FLOOR: MUDSTONE
CACHE ‘The
PSOC 571 DULLCOMPACTCOAL
u*
146.0
200
1313
sampling
~n+wrols and idmtificotionr.
FIG. 1. Drill core log and lithological profile of the Wall seam (PSOC-570-3).
Methods of analysis Each sample was fractionated by extraction first with ammonium acetate solution and then with 1.0 N HCl, as described previously (MILLER and GIVEN, 1986). Extracts were analyzed by plasma arc emission spectroscopy and atomic absorption flame photometry. The insoluble residue was ashed at 750” and solubilized by fusion with lithium metaborate, and the resulting solution analyzed as above. Mineralogical analyses were made by X-ray diffraction and infrared spectroscopy on the products of low-temperature ashing in an oxygen plasma (MILLER et al., 1979). Pyrite contents were determined by the ASTM standard method (ASTM, 1976).
and Engineering, Inc., Charleston, West Virginia. They were then calculated on the dry, mineral-matter-free basis using the mineral matter contents (Table 2) de-
* 9.5 i-l
-’ .1
T
ROOF, UNCONSOLIDATED
$
UPPER
-
BENCH
BANDED
SAND
(PSOC
WOODY
413)
LIGNITE
G
Basic compositional data The elementary compositions of composite intervals of the coals seams are shown in Table 2. The raw elemental data were obtained by Commercial Testing
MIDDLE
I
2
I
LOWER
BENCH
BENCH
outcrops in southeastern United
(PSOC
414)
415)
FLOOR, CARBONACEOUS
FIG. 2. Paleocene-Eocene States.
(PSOC
CLAY
FIG. 3. Lithological profile of columnar section of the Darco lignite seam (PSOC-4 12).
K, N. M~llrr and P. H. Given
1314
52
ROOF CALCAREOIJS, SAND
-5
-
GLAUCONITIC
**
ROOF
II
DARK
Ilt
LIGNITE PYRITE
_____._.
_-
,B
=
54
GREY
MICACEGUS
-101
FINE
iANG
WITH ABUNDANT NODULES
._.
“CLAYEY” (GRADES
._
LIGNITE INTO PARTING.
-
I CM ----..-.
PYRITE
t.ENY _--_I_.
PYRITE
LENI
CARBONACEOUS CLAY PARTING
=
60
LIGNITE NO VISIBLE
--O’ -I
PYRITF
CM
. FLOCR =
72
GREY
CLAY
WITH
ROOT
CASTS
FIG. 4. Lithological profile of a llgnlte dnll IUX~(X‘H-11 I from Western Alabama.
-- I CMPYRl.llC CLAYEY HICACEGUS SAND the co!.,n:n ,nd,cote thP i-ii.
’ Thenumberr a,ongs,de
,. I.C,a
1’~;.5. Llthological profile of a Ignite dnll iitr,’ 5’ ii--r. ‘.I loom Eastern Alabama.
rived from the ash yield and pynte contents. usmg a modified Parr formula (GIVEN and ~‘j\RznH. 197X). However. this formula is not altogether satisfactory for Iignites; when the content of ash-tbrming c’onstiturnts is high. the calculated mineral matter content can he in error by perhaps l-5’7. Hence there is some uncertainty about the contents of organic elements on a dr->. mineral-matter-free basis. partrcularlv oxygen (csti. mated by difference). It is clear that the eastern Alabama. Darco and Wall samples are of similar organic composition and ranh. while the western Alabama sample is of much lower rank. This latter coal has very high contents of hoth pyritic and organic sulfur as seen in Table 3. The ver;, high content of sulfate in the roof may well indicate that the strata have been extensively weathered. The eastern Alabama seam is also rich 111sulfur, but. surprisingly, nearly all the sulfur is in organic combination. The Darco and Wall seams are of low sulfur contrn ts and evidently formed in fresh water conditions. Corresponding petrographic analyses (0 I TZI IS. 1978) are also shown in Table 3. Liptinite macerals are low in all samples. while macerals of the inertimtc suite range up to 20% or more. The relative amounts of macerals of the huminite group (with differing amounts of cellular preservation) are quite variable between the different seams. but woody tissues in the
Alabama samples evidently dation before burial.
underwent
mucI: degn-
RESUL’I 5
The work is to be discussed here and m a suhsequcrrr paper involved taking many thousands ot’datd points Interpretation of the mass of information L;~~ ire
S.mpl e designation
Wdll
seam PSa: 571 PSOC 572 PSOC 573
u 1,s 2.7.
I.5 2.7 1.9
15.1 7.3 22.9
:‘. : k ‘1.I i i 0 08,.,~ i I.. ,i
‘4.1 73 74 !:
Inorganic elements in lignites. II. Table
3.
Petrographic Seam
and
Sulfur
Analyses
of Major
Sections
InterSample designation Ala., west
Val below roof,
m
Forms of Sulfur (5 as % of coal) pyr. SOI org. tot.
Petrographic
Anal..
vol. %,d.m.m.f. ulm. hdet. lip. in.
A B
0 - 0.9 1.2. 1.4
3.0 1.4
3.2 2.8
4.5 4.8
10.7 8.9
47 57
33 28
2 1
18 14
Ala., A east 8 (SCH. C 67A) D E F
0 0.3 0.8. 2.6 2.6- 4.4 4.4. 6.3 6.3. 8.1 8.1-10.5
0.4 0.5 0.6 0.7 0.8 1.9
0.6 0.2 0.1 0.2 0.2 0.5
5.4 3.7 3.9 3.8 3.9 3.1
6.4 4.4 4.6 4.7 4.9 5.5
24 26 20 23 29 26
53 55 57 52 53 47
6 2 3 3 2 2
17 17 20 22 16 25
Barco Lignite PSOC 413 PSK 414 PSOC 415
0 - 1.9 1.0. 1.9 1.9- 2.8
0.6 0.1 0.1
0.1 0 0
1.2 0.9 0.8
1.9 1.0 0.8
62 62 51
27 18 32
2 2 3
9 18 14
Wall Seam PSOC PSOC PSOC
0 - 1.5 1.5. 2.7 2.7. 2.9
0.5 0.3 0
0 0 0
0.6 0.1 0.3
1.1 0.3 0.3
72 87 91
6 5 5
2 1 2
20 7 2
571 572 573
*Names of maceral suites are designated as fallows: ulm = ulminite + gelinite; hdet = humodetrinite; lip = lipinite suite; in = inertinite suite.
rived from a representative selection of plots of element concentration vusus depth. All of the raw data can be found in the Appendix to a thesis (MILLER, 1977) and a Report based on this thesis (MILLER and GIVEN, 1978).
Mineral distributions The yields of ash from acid-insoluble fractions show considerable variations with depth in the various cores. In the Wall, insoluble inorganic matter (as measured by ash yield) tends to decrease fairly steadily with depth from the upper margin of the seam, while in the Darco and Alabama seams it tends to be highest near the upper and lower margins. In contrast, the total content of extractable ions (as measured by the difference between ash yield from the whole coal and from the acidinsoluble part) does not vary much with depth in any of the seams, and constitutes as much as 50-80% of total inorganics, a notably high range. Of course, different phenomena govern the contents of the two kinds of inorganic matter. The input ofboth siliceous particles and cations in solution is adventitious, depending on source and ease of transport, which can change during the accumulation of one peat bed. However, the amount of particulate matter retained depends also on the filtering ability of the peat, not having a defined upper limit, whereas the uptake of cations is limited by chemical equilibria. These points perhaps account for the differing trends of the two kinds of inorganic matter. The changes in pyrite content are illustrated in Fig. 6. Pyrite content is low or very low in the Wall and Darco seams, except for short intervals in the middle of the seams. Large nodular pyrite accounts for 6.5, 39, and 24% of the sediment in the roof section of the Wall, the parting (SS) and the top section of the coal (T 1), respectively; a post-burial origin seems likely, the greater part of the seam evidently having been deposited in fresh water zones of the fluvial/deltaic environment.
1315
In the west Alabama seam, the pyrite content is high and very variable in the upper benches, but quite low in the lower 50-60% of the seam; organic S is about 4.7% throughout. On the other hand, in the east Alabama seam, the content rises from about 1.2% of dry sediment in the upper half of the seam to 1.5-8.5% in the lower half. It should be noted that the organic sulfur content of this seam averaged 5.4% of dry coal in the top 30 cm and was close to 3.8% throughout the rest of the 10 metre profile. Evidently the original peats were sites of intense growth of sulfate-reducing bacteria and variable iron content, as is now the case in the coastal mangrove swamps of the Florida Everglades (GIVEN and MILLER, 1985). All of the profiles show fairly sharp maxima, but these do not coincide with the presence of mineral partings. The sediments in the Porter’s Creek Formation, which probably constituted coastal highlands when the west Alabama seam was accumulating, are known to be rich in siderite and limonite. Leaching of either or both of these minerals from the Porter Creek sediments could have been the source of the pyritic iron in this lignite and its roof rock. A number of minerals were identified by X-ray diffraction in the roof and floor rocks, the partings and in the low temperature ash from the lignites, and the results present some points of interest. In the roof and floor surrounding the Wall seam, but not the coal itself, alkali feldspar was found in addition to other silicates. For a feldspar to reach a sediment more or less intact, rapid transport from the source rock and fairly rapid burial are required. These conditions could well have been met in the intermontane Powder River Basin where the Wall seam occurs. That feldspar was nevertheless not found in the coal suggests that there was some decomposition mechanism operating with the peat; it is known that organic acids, particularly those with chelating ability, can leach ions out of silicate minerals (EHRLICH, 1981). Much more kaolinite was found in the coal of the Darco seam than in the roof or floor. The roof is mostly quartz, and the floor principally a mixture of quartz, kaolinite and illite, while a variety of clays was found in the parting. The relative abundance of kaolinite in the lignite suggests that it was generated in situ by alteration of other clays, through leaching of cations by organic acids. Other evidence supporting the same interpretation will be presented later. Some minor minerals were identified in a heavy gravity fraction of the roof rock (s.g. 2.85): rutile. zircon. tourmaline. hematite and ilmenite. The minerals in roof, lignite and floor of the east Alabama seam were principally quartz and kaolinite, but quartz was much more dominating in roof and floor and was there accompanied by muscovite (a potassium aluminosilicate). The minerals in the coal tend to be concentrated near the upper and lower margins of the seam. Intense low-angle x-ray scattering by the low temperature ash from the west Alabama lignite made
K N. Miller and P. H. Given
FIG.6. Profiles of’ pyrite contents tn tour seams’ 131 I)arco. th) West Alabama. (c) East Alahuma. id! ‘A .!i,
identification of clays difficult, but the parting and floor were shown to consist mostly of various clays. The roof is quite different; it contains abundant oval grain> of glauconite, aragonitic shells, many forms of pyrite. quartz grains, and minor amounis of calcite and ilk. Glauconite, an iron-potassium aluminosilicate. :s thought to be formed in shallow marine waters under reducing conditions (CLOUD. 1955: KKUMBEIN and SLOSS, 1963). It has a high magnetic susceptibility. and this was used to isolate some grains of the mineral and confirm its identity by SEM with stimulated x-m! emission.
The elements that were to an appreciable extent es-. tracted by ammonium acetate solution were <‘a. Mp. Sr. Ba, Na. K and Mn. The ranges of concentration found, the mean values and the covariance (ratio ot standard deviation to the mean value, expressed as ~1 percentage) are shown in Table 4. In general, the concentrations of the elements named tended to van in
Sample
Darca
I?
Range(5llii. 6870 Mean
6400
c0v.z 14
7
Range MeaIl
7910. '10300 a700
express
in ppm
CO”.%
l
**
rangeand
AMMONIW
ACEln!E
iOLuBU
No. of
iignite Wall seam
a parallel manner m any one seam. Ho\lc\:-r ti;~:are appreciable differences in concentmk.)l~ ;tnri 1:’ the Ca/Mg ratio. between seams. (Figs. 7 li ;iit)\s cx amples.) Ca is particularly high (mean val~i i II, i>j drl coal) in the east Alabama heam, WI~I-Iiii>:. ii~i&ci: tluctuation with depth. Overall, the dilkrencts hctwczn seams are no doubt due to local differences ;;I g;ourtii or surface water composition. While clays do contain exchangeable iaucjns. !IIL concentrations of all ammonium-acetate-exli’act~tisl~ cations drop drastically as one passes through rhc margin of the lignite seam into the roof or floor !ocL. and in partings, confirming that the catlons arc pnnclpal!:, attached to the organic matter. as carboxyla~c salts. Ii will be seen that there is some fluctuation ~$conceri tration with depth. but uniess there is a parung:. j ~iiut_-
meant
4
ii,-
.%+ is- 0, is ,i,: 4" I , 1. ,~ 2’
IDlO1340 1250
i,>V L4D ;'i5
33c 240
27'8 230
2370. 3220 2720
Ill& 260 70;
3X750 41 0
10301180
7
:< 17
11
of dry
‘1
coal
,uc150
31;. hi
i 1In ,& i,‘T 3 ‘I
0
90
180
270 360
PPMOFDRYSEDIMENT
0
40
81
IZI! 16" OK
PPMOFDRYSEDIMENT
(/II;L itiJ Sd 3 ',
=':. OFDfi* SEDIMENT
FIG;.7. Distribution of some Ion-rxchangeahic ihc Darco Eeam
&wt.ni\
t!!
Inorganic elements in lignites. II. AMMONIUMACETATESOLUBLE CALCIUM
1317
Table 5.
MAGNESIUM
SOoIUm
Distribution of the Major Alkali and Alkaline Earth Metals and Maganese in the Chemical Fractions
Sample designation
143.0
Amman. acct. ttC1 sol. sol.
Ala.,
Ca
92
7
west (7)*
Mg Na K Sr Ba
79 79 5 91 **
32 2 7 **
Mn
68
25
Ala., east
Ca Mg
91 a8
8
(x)*
; Sr Ba Mn
;; 95 ** 42
: ** 47
Ca Mg Na K ;;
35 88 77
10 4 3
Mn
A? ** 58
; *t 38
Ca Mg
77 90
18 8
143.5-
144.0
144.5-
145& Darco Lignite
145.5
(17)* ‘14b.m + 0.050.300.55 0.601.050.00 0.090.1s0.27 0.360.000.040.080.120.16 4OFDRY
SEDIMENT
%OF
DRY SEDIMENT
%OFDRY
SEDIMENT
FIG. 8. Distribution of some ion-exchangeable elements in the Wall seam.
Wall seam (14)*
* do not vary much (except for the west Alabama core, not shown). The ratio, Ca/Mg, is appreciably different in the coal seams compared to the adjacent strata. Thus is is 5-6 and 1.8 in the roof rocks of the east Alabama and Darco seam respectively, and 9 and 2.9 within the seams. Throughout all seams, the ratio of Ca/Mg is higher in the coal horizon. This finding is consistent with the Ca/Mg adsorption results obtained on the North Dakota lignite (discussed previously; MILLER and GIVEN, 1986), where it was shown that Ca was enriched in a lignite relative to the concentrations in a mixed Ca + Mg solution used for ion exchange in a laboratory experiment.
AMMONIUMACETATESOLUBLE CALCIUM
MAGNESIUM
4
STRONTIUM
f
0.0 0.7 1.4 2.1 2.8 0.000.04 0.080.12 0.16 0 5 OFDRY
SEDIMENT
XOFDRYSEDIMENT
60
%OFDRY
FIG. 9. Distribution of some ion-exchangeable the Eastern Alabama seam.
,
120 180 240 SEDIMENT
elements
in
**
NK" Sr Ba Mn
;: 54 60 49
Acidinsol. Residue 1 ;; 93 1 ** 7 :
z
: 14 18 48
s: 0 ** 11 4 2: a4 5 ** 4 5 ; 27 32 22 3
number of sections used in the analyses
acid-soluble data unavailable for calculating totals
The coal insoluble in ammonium acetate was extracted with 1 N HCl, which will dissolve soluble or reactive salts (carbonates, some sulfides and sulfates) and certain iron oxides (limonite, hematite, goethite), and will solubilize metals from many coordination complexes, including some chelates. Mean relative concentrations of seven elements in the various fractions of the four lignites are shown in Table 5. Most of the Ca, Mg, Sr and Na in the coals is indeed ion exchangeable. A substantial part of the Mn, and of the Ba in the one case for which data are available, are also extracted by ammonium acetate. Only the Wall seam has much of its K in ion exchangeable form; this coal also has less of its Sr exchangeable than the others. There appear to be small amounts of calcite or other carbonate in the seams, but Ca really calls for no further discussion. There is some acid-insoluble Na in the east Alabama and Wall seams. The sodium in the associated inorganic horizons is almost entirely insolubie in acid and the content is low (ranging from 0.009 to 0.56%). The alkali feldspar in the roof and floor of the Wall seam will hold some Na, but otherwise, and in the coals, the insoluble Na will be in clays. The profiles of ion exchangeable Sr and Ba in the Wall seam (Fig. 10) are similar to those of Ca and Mg in the same seam (Fig. 8), indicating the same mode of occurrence. However, significant concentrations of these elements were found also in the acid-soluble and acid-insoluble parts of the seam, with profiles very dissimilar to those of the elements soluble in ammonium acetate. This appears to be a general phenomenon for all seams. The acid-soluble ions are no doubt present as carbon-
ates (barium carbonate was found in the Hagel scan in North Dakota; MILLER and ‘&\,I-Y. 1986). .Icidinsoluble Ba is likely to be in harite. possibl) SI ;I\ sulfate as well: no acid-soluble sulfates were tbunJ The acid-insoluble metals tend in the Wall seam to hc confined to the lower part of the seam t;>r come uncx plained reason. A typical example of the distribution of Mn 11~the fractions is seen in Fig. 1I in 311timi- seams. tlli amounts ofMn extractable by ammonium acetate and by HCI are roughly equal. and between them the twcl fractions account for 90% or more of the element pres ent in the coals (Table y). In the partings, roof and floor rocks, however. both these fractions are vq small, and acid-insoluble Mn dominates (presumabl! as MnO,). The similarity of the distributions of Mn IG the two soluble fractions suggests that the acid-soluble part is coordinated to organic matter. ;I\ ;t transition metal. Mn does form stable orgamc complexes (i’ij; TONand WILKINSON. 1980). However. S\\“\r’\it ( 19x6~. while agreeing that some MI] in Australian brown coals is present as carboxylatc salts. believes that the PI inclpai inorganic forms ofthe element in coals are as irnpuritic\ in calcite, siderite, and sphalerite. all ofwhizh arc ac~tl soluble. In order to discuss acid-insoluble K and Mg. some remarks about Si and Al must bc ma& as well. 45 expected from the mineralogical analyses. all ot the SI and much of the Al are in the acid-insoluble lYact~an\ where they are combined as silicates. I‘hc profiles of all four elements are more or less parallel, and there are correlations of acid-insoluble K and Mg with the content of clays, confirming the obvious suggestion that the metals are tightly bound in clay minerals. as f’o~ example, the K in illite and mixed-layer clays. An cxample of a set of profiles is shown in Fig. 12. where the markedly parallel variations can be readily seen. The ratio, K/Al (acid-insolubles), is also shown In Fig. 12 for the Darco seam. This ratio serves as a useful index of the type of clay present. It is always higher in the primarily inorganic horizons. but very low within
V>‘I.? I !li the roof. 0.2U in the parting and 0. I6 in the liuor rock, but never more than 0.07 in lhl: coal. in !II;, i.1~3 .tiabama seam acid-insoluble Al is i .Y- ~o!‘tl~ wcl~nwr~: III both roof and coal. indicating a near-l! *_onstaIii c.ontent of clay, while the amount of k 111thcsc &I> minerals is quite different. the K/Ai ram.’ cfroppln$! from about 0.5 in the roof to 0.05 in the co‘i‘ it seett~s reasonable to suppose that the delrn:il ITI~!I crals deposited in and immediately over a SG~I wuu! t)rganismz would have been present. 1x11 tot III lirc primaril> inorganic zones. Thus e.xtcnsi\e IUS of h: and ti)rmation I>[authigenic kaulinitz took pijc,c. i lx phenomenon w_vn~ to be quite general 111:ill of’ tlrt* ,eams studie~i the coal. 3’hus. In the Darco scam. the ratIt
AMMONIUMACETATE SOLURLF AC!!: V;’ i’y’ E
li, :I! INK)L!,B, t
4460 ii u I8 36 54 12 0 ?? is 0 I8 36 54 72 0 PPM OF DRY SEDIMEKT PPM OF DRY SEDIMENT PPY W DRY SEDMEN!
Inorganic elements in lignites. II.
9 OF DRY SEDIMENT
“. OF DRY SEDIMENT
5 OF DRY SEDIYEHT
1319
WY OF DRY SEDIYEN,
5 OF DRY SEDMEKT
FIG.12. Distribution of some acid-insoluble elements in the Darco lignite seam.
Acid-soluble Al, Ti and Fe The change in the degree of acid-extractability of Al, Ti and Fe between the organic-rich and the inorganic-rich horizons of all four seams provides a great deal of evidence that the form and distribution of these elements is markedly affected by interactions with organic and biological processes in the coal bed in a manner not appreciated hitherto. The interpretation is not identical for all three elements, but the important phenomenon is observed with all, that the fraction by weight of Al, Ti and Fe that is extractable by acid is always much higher in the organic horizons than the inorganic, regardless of the absolute amount of the elements present. As examples, Fig. 13 shows the profiles of acid-soluble Al in two seams. Thus the concentrations in roof and floor are about 0.0 1 and 0.06% respectively in the east Alabama seam, and 0.12-0.25% in the organic layers. In fact, acid-soluble Al can account for up to 50% of the Al in a seam. In no case is there a correlation between acid-soluble and insoluble Al, contrary to what one might have expected if some Al were leached from clays during extraction with HCl. It therefore seems likely that acid-soluble Al is complexed by organic matter (how this might happen is discussed below). In previous work with a North Dakota lignite (MILLER and GIVEN, 1986) the greater part of the acid-soluble Al was found in the fractions of lower specific gravity, thus excluding the possibility that the extractable Al is in the form of hydrated oxides, and hence the same conclusion was drawn. The breakdown of detrital clays by organic matter, discussed above in connection with K, could provide a source of Al in soluble or colloidal form. The complexation of Al with humic acids has been reported by LAKATOS et al. ( 1972). A word of explanation is perhaps needed for the inference that some metals are present in lignites as coordination complexes with organic ligands. In the nature of things, one cannot isolate and identify coordination complexes in coals. One can only draw in-
ferences from the behavior of elements in physical or chemical fractionation and the known chemistry of the element. Three types of compound known to form many chelate coordination complexes are salicylic acid (I), I-hydroxy-9, IO-anthraquinone (II), and catechol (III).
COOH
0 H\O
&OH q$
OH
&OH
0 I
II
III
A proton is lost as a metal ion takes its place, bridging the two functional groups. We envisage such structures forming part of the lignite macromolecule. Complexes of II with Al+++ and Fe+++ among others, are known to be easily formed and particularly stable. Examples of Ti distributions are shown in Fig. 14 for the east Alabama and Wall seams. As can be seen, the acid-extractability of Ti in the roof, floor and partings constituting the inorganic horizons of the seam is very low, while in the organic horizons it is generally much higher, tending to peak near the margins. Considering all four sites, l- 10% of Ti present is extractable by acid from the inorganic horizons, but 30-60% in the coals. None of the common Ti minerals is soluble in dilute mineral acid. Also, leaching of Ti from clays must again be ruled out as a major factor. In all cases, the acid-insoluble titanium remains high in the roof and floor, but tends to be low in the coal. We infer that the acid-soluble Ti is organically complexed, and that the acid-insoluble in the inorganic rocks is present in mineral forms. The same may be true of the insoluble Ti in the coals, but not necessarily. It should be noted that KARNER et al. (1986) do not report acid soluble Ti in the two North Dakota seams they examined.
K. N. Miller and P. H. Given
133)
ii i1
i
64
-1-7
0.00
0.06 % OF DRY
0.08
0.12
0 16
$---t---. 000
0 06
0.12
0.1.9
0 24
% OF DRY SEDIMENT
SEDIMENT
FIG. 13. Distribution of‘acld-soluble aluminum rn the Darcc~ lignite and eastern .4labama seam<
In our study of the North Dakota lignite MILI.I K and GIVEN, 1986). it was found that most of the acidsoluble Ti occurred in the fractions of lower specific gravity, but so also did an appreciable (though lowerj proportion of the acid-insoluble. We therefore postulated that some of the acid-insoluble Ti was organicall> bound. citing the work of ESKE.NKC~ ( I Y72), who studied the adsorption of Ti on peat humlc acids and coals in the pH range 0.X-2.0. and found that oni\
part was desorbed in HCI of p1-l I .4. In lhc PI-esent work we did not perform gravity fractionations and SC> have no evidence to show whether some of !bc XXinsoluble Ti in the lignites is organically h& 4 plausible hypothesis explaining hog +r’gan~ complexation of Ti could come about can be proposed. The mineral ilmenite (FeTQ\ often OCCUI-L ii: sand:. and it was in fact detected in a heavy grabi~; !i’actthr. of. the sandy roof of the Darco scam. NW I[ 15Lnw F that, in mildly acidic and reducing conditi
DISCUSSION
AND
CON(‘I.1 !-dONh
.4s was stated earlier, the prtncipal object1 Ws of’thrs study were to determine how major, minor
..
-,
q
,631, 18Lr 364
54”
PPM OF DRY SEDIMENT A
EGTERN
72”
ooooii
022
033
” OF DRY SEDIMENT nLPlBn~n
IE&M
03L
r
0
110
220
-
330
7
7
440
00””
PPM OF DRY SEDIMENT u
Fii; 14. i>istribution t>ftitanium In two hgnites
-
I”
,,A
/’ 4:‘
. OF DR? SEDIMEN1 WALL
StAa
I’
Inorganic elements in lignites. II.
it can be said now that this also is much influenced by the fact that we are dealing with an organic sediment. The phenomena discussed seem to be quite general for peats and lignites, though the magnitude of effects varies from site to site. Vegetational changes within and between peat-forming ecosystems are indeed significant and will, no doubt, in part determine the inorganic signature remaining within the preserved organic matter. However, the pH of the connate water in peats is variable and will obviously have a number of effects on solution equilibria and hence on the fate of many of the more mobile elements. Ionic strengh, which varies greatly between different peats, likewise has a considerable effect on rates of diffusion of ions and even on rates of bulk flow of water (GIVEN and DICKINSON,1975). Some water in peat is “trapped” and not available at all for diffusion. The amount and nature of the specific functional groups will also play a considerable role in what inorganic matter is found there. In fact, peats are very complex physicochemical and colloidal systems (GIVEN and DICKINSON, 1975).
DPlRCO SEAM
i
I
I
I
0.140.200.420.56 X OF DRY SEDIYEHT
,
I c 0.36 0.54 D.00 0.18 X OF DRY SEDIMENT
1321
I
072
FIG. 15. Distribution of acid-soluble iron in the Wall and Darco lignite seams.
fractionated a number of the major and minor inorganic elements in lignites on the basis of solubility in various reagents, and have successfully related these solubility data to mineralogical observations and seam profile analyses (of both organic and inorganic zones) to draw inferences on the predominance of organicinorganic interactions. Though the results are not unequivocal in all cases, the methods used have permitted a fair degree of success in achieving the objectives. Some of the interactions inferred have not been discussed previously. Our identifications of ion-exchangeable elements are similar to those of KARNER et al. (1986), though we find greater differences between seams. Our observations and conclusions relating to Ti are different. The most notable observation here and in Part I (MILLER and GIVEN, 1986) is the large extent to which the organic sediment determines what inorganic matter it associates with. Thus substantial proportions of the ash-forming constituents, such as Ca, Mg and Na, are present as carboxylate salts. Some Mn, Al, Ti and Fe are most probably held as coordination complexes by the organic matter. The generation of pyrite stems from bacterial respiration, which requires rather specific organic compounds as fuels, and these are probably formed by fermentative processes of other organisms, such as Clostridium sp. (BELYAEVet al., 1981). Carbonates are formed in situ largely as a product of aerobic respiration of roots and microorganisms. Organic acids in the sediment cause significant alteration of clay minerals and degradation of accessory minerals like ilmenite and feldspars. The distribution of trace elements will be discussed in a subsequent paper, but
Lignites no doubt will inherit most of the results of phenomena in peats, but are themselves subject to alteration by post-depositional processes involving the movement of ground water through the seams. The relative contributions of peat and post-depositional phenomena, particularly with regard to highly mobile elements, is difficult to define though this study has shed some light on the importance of both. The study discussed here and earlier (MILLER and GIVEN, 1986) has been almost entirely concerned with the intrinsic nrocesses in peats and lignites, and these are of course of great importance in determing what suite of inorganic materials a lignite will eventually contain. Other important determining factors are the extrinsic ones-that is, the nature of the rocks being eroded around the coal-forming basin and the local hydrology as it affects transport of mineral grains and ions in solution during and after peat formation. These additional factors require a separate study, which we have not made, though such a study would be an important complement in seeking to understand the inorganic geochemistry of coal-forming systems. KARNERet al. (1986) and several other authors have reported that some major and trace elements and/or minerals tend to be concentrated near the margins of seams. There seem to be disagreements on which elements and on the underlying reasons. We shall discuss this matter with our own data in Part III of this series of papers.
Acknowledgements-This project was supported in part by the U.S. Dept. of Energy under Contracts E(49-18)-2030 and E(49- 18)-2494. The coal samples and their basic characteristics were drawn from the Penn State Coal Sample and Data Base, assembled under the direction of Dr. William Spackman. The authors are indebted to Mr. Norman Suhr and Ms. Marion Briscoe for analyses by emission spectroscopy.
Editorial handling: J. W. de Leeuw
R N. Miller an
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